This invention relates generally to nuclear reactors and more particularly, to methods and apparatus for securing pressure lines to jet pumps in a nuclear reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
Boiling water reactors have numerous piping systems, and such piping systems are utilized, for example to transport water throughout the RPV. For example, core spray piping is routed over and through the shroud to deliver water to the reactor core.
Jet pump diffusers in a nuclear reactor, such as in a boiling water nuclear reactor, typically form part of a jet pump and are utilized to maintain the floodability of the reactor core at a safe level. The jet pump diffuser also typically discharges water from an inlet mixer into the lower core plenum to maintain adequate recirculation flow.
One known jet pump diffuser has a generally conical shape, and the larger diameter end of the diffuser is welded to an adapter assembly engaged to an INCONEL nickel-chromium alloy shroud support plate. The smaller diameter end of the diffuser forms a slip fit connection to the inlet mixer section of the jet pump.
Water, generally under high pressure, flows through the jet pump diffuser from the smaller diameter end to the larger diameter end. The water exits the diffuser at the larger diameter end and is discharged through the adapter assembly into the lower core plenum. The discharged water mixes with the water in the lower core plenum and may cycle through the reactor.
As the water flows through the jet pump diffuser, the speed of the water decreases due to the increasing diameter of the diffuser. Decreasing the water speed and pressure just prior to discharging the water into the lower core plenum improves the mixing and flow characteristics of the discharged water.
For safe operation of a nuclear plant, the pressure drop across the diffuser is continually monitored. For this purpose, small sensing lines are mounted to the exterior of the diffuser, and communicate through the walls of the diffuser so that the pressure differential may be sensed.
Under certain conditions, the pressure sensing lines may vibrate. This vibration subjects the stainless steel stand off blocks mounted between the diffuser and line to sever between the block and line. This severance of the line from its mounted stand off block is an undesirable condition.
T-Bolt clamps have been developed that clamp the diffuser sensing lines to the diffuser providing added support and minimizing vibration. To secure the T-bolt clamp to the diffuser, slots need to be machined into the diffuser so that the T-bolt can be inserted into the slot. The T-bolt clamp is then secured around the sensing line to prevent excessive vibration of the sensing line. The slot must be positioned behind the sensing line. Typically such slots are burned or machined into the diffuser by using an electrode discharge machining (EDM) process. A major problem to overcome is burning the slot into the diffuser behind the sensing line without burning and damaging the sensing line at the same time.
It would be desirable to provide a EDM tool for burning slots or holes in the diffuser behind the sensing line that will not damage the sensing line during the EDM process.